B PRINCIPLES OF ORGANIC MEDICINAL CHEMISTRY (ii)Allopurinol competes with hypoxanthine for xanthine oxidase (i)Physostigmine and neostigmine compete with acetylcholine for cholinesterase 4.Through Receptors A large number of drugs act through specific macromolecular components of the cell, which regulate critical functions like enzymatic activity,permeability,structural features, template function etc.These macromolecules,which bind and interact with the drugs,are called receptors. DRUGRECEPTOR INTERACTIONS Introduction The conc ggested at the end of the 19th and the be nd La ley both contributed the ides that compounds displaved biological activity by binding to cellular constituents (Ehrlich:cor. pora non agunt,nisi fixata',which tells us that 'agents do not work,unless bound')that were soon named'receptors'(Langley:receptive substances).One could consider that every protein that acts as the molecular target for a certain drug sh nitiates the c em Drug-Receptor Complex Nomenclature 1.Agonist roper drug that activates a receptor is knows as agonist,which has following Agonists can differ in both affinity and efficacy for the receptor High efficacy agonists are full agonists because they elicit maximal effects vefficacy agonists are partial agonists because they cannot elicit a maximal effect nigh concer fals t(the neurotransmitter,its 2.Antagonist- A drug that does not activate the receptor is antagonist,which possess the following features .Antagonists also prevent the activation of the receptor by an agonist,thus antago- nists are essentially zero efficacy drugs Competitive antagonists bind to the ame binding site as the agonist and therefore comnete with the agonist for that binding site ·Non nists have a different binding ago ith the ists have a bind ing site within the ion channel associated with the receptor complex. Chemical Nature of Receptors 4a1 ntil in the 1970s the de and antification of binding si for drugs in tissues or isolated cells.Nowadays,structural information (X-ray,NMR)of a
8 PRINCIPLES OF ORGANIC MEDICINAL CHEMISTRY C-8—N-CHEMI\CHE2-1.PM5 (iii) Allopurinol competes with hypoxanthine for xanthine oxidase (iv) Physostigmine and neostigmine compete with acetylcholine for cholinesterase. 4. Through Receptors A large number of drugs act through specific macromolecular components of the cell, which regulate critical functions like enzymatic activity, permeability, structural features, template function etc. These macromolecules, which bind and interact with the drugs, are called receptors. DRUG-RECEPTOR INTERACTIONS Introduction The concept of proteins as drug targets is not novel and was suggested at the end of the 19th and the beginning of the 20th centuries. Ehrlich and Langley both contributed the idea that compounds displayed biological activity by binding to cellular constituents (Ehrlich: ‘corpora non agunt, nisi fixata’, which tells us that ‘agents do not work, unless bound’) that were soon named ‘receptors’ (Langley: ‘receptive substances’). One could consider that every protein that acts as the molecular target for a certain drug should be called a receptor. A receptor is a component of a cell or organism that interacts with a drug and initiates the chain of biochemical events leading to the drug’s observed effects. Drug-Receptor Complex Nomenclature 1. Agonist—A drug that activates a receptor is knows as agonist, which has following properties; Agonists can differ in both affinity and efficacy for the receptor High efficacy agonists are full agonists because they elicit maximal effects Low efficacy agonists are partial agonists because they cannot elicit a maximal effect at receptors even at high concentrations (false transmitters) Direct agonists act on receptors, while indirect agonists facilitate the actions of the endogenous agonist (the neurotransmitter, itself) 2. Antagonist—A drug that does not activate the receptor is antagonist, which possess the following features ; · Antagonists also prevent the activation of the receptor by an agonist, thus antagonists are essentially zero efficacy drugs Competitive antagonists bind to the same binding site as the agonist and therefore compete with the agonist for that binding site Non-competitive antagonists have a different binding site to the agonist and therefore do not compete with the agonist. Some non-competitive antagonists have a binding site within the ion channel associated with the receptor complex. Chemical Nature of Receptors For many years receptors, remained hypothetical structures, until in the 1970s the development of radioactive ligand led to the visualization and quantification of binding sites for drugs in tissues or isolated cells. Nowadays, structural information (X-ray, NMR) of a
GENERAL PRINICPLES OF DRUG ACTION 9 lar proces Receptors are no longer hypothetical.Hundreds of receptor poteins have been isolated, purified,cloned,and their primary amino acid sequence has been established.It has been possible to study the receptor by binding assay,biochemical characterization,immunological characterization and molecular biological characterization. Most of the receptors like regulatory enzymes (dihydrofolate reductase enzyme) acetylcholinesterase transport proteins and structural proteins (Tubulin)are protein in na- ture and some are glycoproteins(G-protein coupled receptors)or nucleic acids. Types of Receptors However.the overall structure of receptor proteins is often not so div that signal transmission via receptor proteins is governed by a limited number of basic mecha nisms that are utilised in an extre emely efficient way.One distinguishes four super-families of receptor proteins,which cover most of the relevant receptor proteins.These four receptor families are edionchannels.Liga ae nd-gated ion channels,which are membrane-bound cep d t an e les ion own as i the GABA-A receptor NH NH HO. MeCOOCH,CH,N'Me Acetylcholine (-ymn) HNCH,CH,CH,COOH GABA (Am outyrie acid Thrombin Structures of selected ligands for G-protein coupled receptors (ii)G- e nucleotide. regulatory protein)e G are mem receptors coupled G-prote
GENERAL PRINICPLES OF DRUG ACTION 9 C-8—N-CHEMI\CHE2-1.PM5 variety of receptor proteins is known and this has led to the development of detailed insights in the molecular processes involved in drug–receptor interactions. Receptors are no longer hypothetical. Hundreds of receptor poteins have been isolated, purified, cloned, and their primary amino acid sequence has been established. It has been possible to study the receptor by binding assay, biochemical characterization, immunological characterization and molecular biological characterization. Most of the receptors like regulatory enzymes (dihydrofolate reductase enzyme) acetylcholinesterase transport proteins and structural proteins (Tubulin) are protein in nature and some are glycoproteins (G-protein coupled receptors) or nucleic acids. Types of Receptors However, the overall structure of receptor proteins is often not so divergent, suggesting that signal transmission via receptor proteins is governed by a limited number of basic mechanisms that are utilised in an extremely efficient way. One distinguishes four super-families of receptor proteins, which cover most of the relevant receptor proteins. These four receptor families are: (i) Ligand-gated ion channels. Ligand-gated ion channels, which are membrane-bound receptors, directly linked to an ion channel. They are also known as ionotropic receptors. Examples include the nicotine acetylcholine receptor, glutamate receptor and the GABA-A receptor. MeCOOCH CH N Me 22 3 + Acetylcholine H NCH CH CH COOH 2 222 GABA ( -Aminobutyric acid) γ Histamine serotonin (5-Hydroxytryptamine) Thrombin N N H NH2 HO N H NH2 Structures of selected ligands for G-protein coupled receptors. (ii) G-protein (Guanine nucleotide-regulatory protein) coupled receptors. Gprotein coupled receptors, which are membrane-bound receptors coupled to G-proteins. After
10 PRINCIPLES OF ORGANIC MEDICINAL CHEMISTRY activation of the G-proteins a variety biochemical signal transduction pathways can be acti. vated.Many chemical messengers,like hormones and various neurotransmitters,act through G-protein coupled receptors.They are also known as metabotropic receptors or 7- transmembrane receptors.Ex:Muscarinic acetylcholine receptors and adrenergic receptors. (iii)Tyrosine Kinase-linked Receptors.Tyrosine kinase-linked receptors.are mem brane bound ceptors and contain an intrinsic enz matic function (tyrosine kinase activity) in their intracellular domain.Upon combination with ligand like insulin,the receptor is acti- vated and is able to phosphorylate tyrosine residues of other intracellular proteins.Protein phosphorylation is one of the underlying mechanisms of the regulation of protein function.Ex: Receptors for insulin and various cytokines and growth factors. Kinase-linked G-protein coupled receptors Intracellular steroid receptor C -N DNA binding Sch representation of the four majo for the are localised in the cell membrane.The ligand-gated on channels are made up of an assembly by 4-5 subunits,which each contain four transmembrane domains ()gene transeription:Intracellular receptor regulating gene transcription.which are located in the binding of the a nr ate chemical e.g.steroid hormones.the activated receptors translate to the nucleus and initi ate gene transcription.These are also known as nuclear receptors.Ex:Receptors for steroid hormones,thyroid hormones and vitamin D. Drug Receptor Interactions duce ph ee-point
10 PRINCIPLES OF ORGANIC MEDICINAL CHEMISTRY C-8—N-CHEMI\CHE2-1.PM5 activation of the G-proteins a variety biochemical signal transduction pathways can be activated. Many chemical messengers, like hormones and various neurotransmitters, act through G-protein coupled receptors. They are also known as metabotropic receptors or 7- transmembrane receptors. Ex: Muscarinic acetylcholine receptors and adrenergic receptors. (iii) Tyrosine Kinase-linked Receptors. Tyrosine kinase-linked receptors, are membrane bound receptors and contain an intrinsic enzymatic function (tyrosine kinase activity) in their intracellular domain. Upon combination with ligand like insulin, the receptor is activated and is able to phosphorylate tyrosine residues of other intracellular proteins. Protein phosphorylation is one of the underlying mechanisms of the regulation of protein function. Ex: Receptors for insulin and various cytokines and growth factors. N N Kinase-linked receptors G-protein coupled receptors Ligand-gated Ion-channels N C C C 4X tyrosine kinase Intracellular steroid receptor ligand binding DNA binding C N Schematic representation of the four major classes of receptor proteins. Except for the steroid receptors, the receptor proteins are localised in the cell membrane. The ligand-gated ion channels are made up of an assembly by 4–5 subunits, which each contain four transmembrane domains. (iv) Intracellular receptors regulating gene transcription: Intracellular receptors regulating gene transcription, which are located in the cytosol. Upon binding of the appropriate chemical e.g. steroid hormones, the activated receptors translate to the nucleus and initiate gene transcription. These are also known as nuclear receptors. Ex: Receptors for steroid hormones, thyroid hormones and vitamin D. Drug Receptor Interactions Majority of drugs show remarkably high correlation of structure and specificity to produce pharmacological effects. A minimum three-point attachment of a drug to a receptor site is
GENERAL PRINICPLES OF DRUG ACTION required.In most cases specific chem ary drug str cally change 之aao (i)Covalent interactions.These chemical forces may result in a temporary binding of the drug to the receptor.Frequently,a covalent bond is firm and described as essentially biological conditions.Since by definition the drug-receptor interaction is reverbecalent bond formation is rather rare except in a toic sitution.Eample (a)A covalent bond is formed between the activated form of phenoxybenzamine (a-adrenergic receptor antagonist) ()Antineoplastic or antibiotic drugs act mainly through the formation of covalent bonds (c)The DNA-alkylating chemotherapeutic agents are chemically highly reactive,form (ii)Ionic interactions.Since many drugs contain acid or amine functional groups, which are ionized at physiological pH.Ionic bonds are formed by the attraction of opposite charges in the receptor site with the ionized groups of the drug molecule.They are strong electrostatic interactions(5-10 kcal/mol)and are responsible for relative orientation of the drug to its binding site.Electrostatic interactions tend to be much more common than the covalent bonding in drug-receptor interactions.Attraction between ions of opposite charge is en them.str ng electrostatic inter ccur be en perm have a relative contrib ation. ively high Ex:In acetylcholine molecule,the positively charged quaternary nitrogen may be attracted to the negative charged ionized carboxyl group present in the receptor site. (ii)Hvdrogen bonding interactions (non-ionic/neutral).Polar-polar interaction are the attraction of DD site cha res.The drug-receptor reaction is essentially an exchange of en a drug molecule surrounding water and the tor site The hydrogen bond strength is distance dependent may range from 5-7 kcal/mol,depending on the binding environment. (iv)Vander Waals interaction.These forces have the following characteristic feutures (a)Interactions at a close range (b)The vander waals interaction forces occur less frequently than hydrophobic forces (c)Interactions are much weaker(-0.5-1 kcal/mol)than other electrostatic interaction (d)Close contacts(attractive forces)over a large surface areai.e.at the interface ofligand and binding site.may contribute to binding (Hvdrophobic lipophilic interactions.Finally hydrophobic bonds are formed be tween non-polar hydrocarbon gro ps on the drug and those in the receptor site.these bonds are not very specific but the interactions do occur to exclude water molec les
GENERAL PRINICPLES OF DRUG ACTION 11 C-8—N-CHEMI\CHE2-1.PM5 required. In most cases specific chemical structure is required for the receptor site and a complementary drug structure. Slight changes in the molecular structure of the drug may drastically change specificity. To initiate a biological response, the drug must form bond with the receptor surface. Different types of binding forces that may exist in drug-receptor interactions are as follows: (i) Covalent interactions. These chemical forces may result in a temporary binding of the drug to the receptor. Frequently, a covalent bond is firm and described as essentially “irreversible” under biological conditions. Since by definition the drug-receptor interaction is reversible, covalent bond formation is rather rare except in a toxic situation. Examples: (a) A covalent bond is formed between the activated form of phenoxybenzamine (α-adrenergic receptor antagonist) (b) Antineoplastic or antibiotic drugs act mainly through the formation of covalent bonds (c) The DNA-alkylating chemotherapeutic agents are chemically highly reactive, forming covalent bonds with DNA functional groups. Such covalently modified DNA may be incompatible with successful tumor cell division (ii) Ionic interactions. Since many drugs contain acid or amine functional groups, which are ionized at physiological pH. Ionic bonds are formed by the attraction of opposite charges in the receptor site with the ionized groups of the drug molecule. They are strong electrostatic interactions (5-10 kcal/mol) and are responsible for relative orientation of the drug to its binding site. Electrostatic interactions tend to be much more common than the covalent bonding in drug-receptor interactions. Attraction between ions of opposite charge is inversely proportional to the square of the distance between them. Strong electrostatic interactions occur between permanently charged ionic molecules. The overall contribution of ionic interactions may be overemphasized due to desolvation. Ionic bonds have a relatively high stability. Ex: In acetylcholine molecule, the positively charged quaternary nitrogen may be attracted to the negative charged ionized carboxyl group present in the receptor site. (iii) Hydrogen bonding interactions (non-ionic/neutral). Polar-polar interactions are the attraction of opposite charges. The drug-receptor reaction is essentially an exchange of the hydrogen bond between a drug molecule, surrounding water, and the receptor site. The hydrogen bond strength is distance dependent may range from 5 – 7 kcal/mol, depending on the binding environment. (iv) Vander Waals interaction. These forces have the following characteristic feutures: (a) Interactions at a close range (b) The Vander Waals interaction forces occur less frequently than hydrophobic forces (c) Interactions are much weaker (~ 0.5-1 kcal/mol) than other electrostatic interactions (d) Close contacts (attractive forces) over a large surface area i.e. at the interface of ligand and binding site, may contribute to binding (v) Hydrophobic/Lipophilic interactions. Finally hydrophobic bonds are formed between non-polar hydrocarbon groups on the drug and those in the receptor site. These bonds are not very specific but the interactions do occur to exclude water molecules
12 PRINCIPLES OF ORGANIC MEDICINAL CHEMISTRY Most of the drug molecules have a non-polar portion(alkyl or aryl groups)which may combine with non-polar portion of the receptor site through hydrophobic forces.Hy- drophobic interactions are generally weak,but important.Hydrophobic interactions are probably significant in driving interactions (a)Between lipophilic drugs and the lipid component of biological membranes (b)Between drugs and relatively nonpolar(non charged)receptor regions Receptor site theories After attachment of drug molecule to a receptor site,a drug may either initiate a re- sponse or prevent a response from occurring. This concept can be easily understood if on ligand ds) t fulfil all pen the door(pr can fit in the lock but not pe uently they cannot open the door yet.By fitting into the lock,these keys pre vent the original key from fitting into the lock and opening the door.The concept of receptor antago- nism is extremely important in medicinal chemistry and is very often the underlying mecha- nism of drug action e.g.to prevent the constriction of airway smooth muscle in asthmatic conditions one can administer receptor antagonists that prevent the actions of the signalling molecules causing muscle contraction(e.g.histamine and leukotriene antagonists). effect s perfectly into the lock( tor)and will be able to。 o the a re onse).Th small difference between the two keys is indic by the circle.Th tin the ng in th rd an antagonist as an imperfect key and a receptor agonist as the perfect kev. (i)Occupation theory.In fact,similar mathematical models have been applied to the receptor-ligand interaction.Clark's occupation theory was the first model that could describe the observations of drug action on isolated tissues.In this theory the receptor-ligand interactio is considered tobe a bimolecular interaction,in which the receptor-ligand complex is responsible for the gene e bi ogical effect.Clar assum tha the ene f a drug wer I to the e fraction of eceptors occup rk th (A)in way with the occupy all agon eracts in a the comp AR gives ris to the A+RAR-effect
12 PRINCIPLES OF ORGANIC MEDICINAL CHEMISTRY C-8—N-CHEMI\CHE2-1.PM5 Most of the drug molecules have a non-polar portion (alkyl or aryl groups) which may combine with non-polar portion of the receptor site through hydrophobic forces. Hydrophobic interactions are generally weak, but important. Hydrophobic interactions are probably significant in driving interactions (a) Between lipophilic drugs and the lipid component of biological membranes (b) Between drugs and relatively nonpolar (non charged) receptor regions Receptor site theories After attachment of drug molecule to a receptor site, a drug may either initiate a response or prevent a response from occurring. This concept can be easily understood if one considers the ‘lock-and-key’ principle for ligand–receptor interaction. Only keys (ligands) that fulfil all criteria for fitting into the lock (receptor) can open the door (produce an effect). Some keys can fit in the lock but not perfectly, consequently they cannot open the door yet. By fitting into the lock, these keys prevent the original key from fitting into the lock and opening the door. The concept of receptor antagonism is extremely important in medicinal chemistry and is very often the underlying mechanism of drug action e.g. to prevent the constriction of airway smooth muscle in asthmatic conditions one can administer receptor antagonists that prevent the actions of the signalling molecules causing muscle contraction (e.g. histamine and leukotriene antagonists). effect Lock-and-key principle for receptor–ligand interactions. Only one of the keys (ligands) fits perfectly into the lock (receptor) and will be able to open the lock (give a response). The small difference between the two keys is indicated by the circle. The ‘imperfect’ key will fit in the lock, but is not able to open the lock. By sitting in the lock the imperfect key prevents the perfect key getting into the lock. One could regard an antagonist as an imperfect key and a receptor agonist as the perfect key. (i) Occupation theory. In fact, similar mathematical models have been applied to the receptor–ligand interaction. Clark’s occupation theory was the first model that could describe the observations of drug action on isolated tissues. In this theory the receptor–ligand interaction is considered to be a bimolecular interaction, in which the receptor–ligand complex is responsible for the generation of the biological effect. Clark assumed that the effects of a drug were proportional to the fraction of receptors occupied by the drug. Consequently, for a maximal effect the drug has to occupy all receptors. In Clark’s theory, the agonist (A) interacts in a reversible way with the receptor (R) and the formed complex (AR) gives rise to the effect: A + R AR → effect